I have a SS screw-on freewheel type hub (DMR revolver) which gives me no chainline adjustment, so I need to make sure I get the correct length BB to give me a straight chainline. The cranks I have are ISIS (Raceface Prodigy) and I don't have any old BB's to try. The chainring will go in the outer position on the crankarm. So is there a simple way to ensure I buy the right length BB?

Lewis1641 recently had similar trouble i think and was told you can space the freewheel somewhat.

Like you say though you will put it in the outer position so that means you could also use the inside.
Im tempted to stay start with a 113 but it can be hit and miss and if you did want to run the ring on outer you may need just a bit shorter than what the 'normal' one would be.

The chainring i have only fits in the outer position. I was hoping someone would say that all screw-on freewheel type hubs line up with the outer ring if you run a chainline of say 50mm or something. That way all I'd have to do is find out what length BB gave that chainline for my crank.

"All RF cranks are designed to have a 48.5-49.5mm chainline when installed on a 113mm ISIS crank"
This will be the distance of the middle ring from the centre line of the bike.
Working backwards, you need to find the chainline of the rear sprocket.
By measuring between the axle faces (that butt up to the dropouts) find the midpoint of the hub.
Then measurehow far the sprocket teeth (centres) are from the centre line.
Adjust the length of the BB axle to suit. eg, if the rear chainline is 53mm, the front has to go out to 53 too, so the BB needs to be8mm longer: (53-49) X2 .
This will move the pedals out a bit but it's no big deal.

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